Chromizing ferrous metal substrates

ABSTRACT

IN ORDER TO MITIGATE THE FORMATION OF A LAYER OF SINTERED POWDER ON THE SURFACE OF FERROUS SUBSTATES CHROMIZED BY THE APPLICATION OF AN ADHESIVE LAYER FOLLOWED BY THE APPLICATION OF CHROMIUM POWDER AND A HEAT TREATMENT TO BRING THE FORMATION OF A DIFFUSION ALLOY SURFACE LAYER, HYDRATED MAGNESIUM OR ALUMINUM HALIDES OR MAGNESIUM OXYHALIDE IS APPLIED TO THE SUBSTRATE BEFORE, WITH OR AFTER THE CHROMIUM. PREFERABLY THE HALIDE IS APPLIED AS AN AQUEOUS SOLUTION BEFORE THE CHROMIUM SO AS TO FORM THE ADHESIVE LAYER. PH MODIFIERS MAY BE INCORPORATED TO REDUCE THE ACIDITY OF THE SOLUTION IN THE CASE WHERE ALUMINUM CHLORIDE IS USED AS THE HALIDE.

United States Patent Ofiice 3,778,297 CHROMIZING FERROUS METALSUBSTRATES Kenneth Urmston Holker, Harrogate, England, assignor toNational Steel Corporation, Pittsburgh, Pa. No Drawing. Filed Jan. 3,1972, Ser. No. 215,150 Claims priority, applicatigiwgrleat Britain, Jan.5, 1971,

Int. Cl. C23c 9/00 US. Cl. 117-107.2 P 20 Claims ABSTRACT OF THEDISCLOSURE In order to mitigate the formation of a layer of sinteredpowder on the surface of ferrous substrates chromized by the applicationof an adhesive layer followed by the application of chromium powder anda heat treatment to bring about the formation of a diffusion alloysurface layer, hydrated magnesium or aluminum halides or magnesiumoxyhalide is applied to the substrate before, with or after thechromium. Preferably the halide is applied as an aqueous solution beforethe chromium so as to form the adhesive layer. pH modifiers may beincorporated to reduce the acidity of the solution in the case wherealuminum chloride is used as the halide.

The present invention relates to a process of the type described inapplication No. 35,252 filed on May 6, 1970. In that application thereis described a process for chromizing ferrous metal substrates in whichthere is first formed on the substrate a layer of adhesive material towhich is then applied a chromium-containing metal powder, a halide beingincorporated in the coating either as part of the adhesive layer orseparately, and the coated substrate is then subjected to a heattreatment to bring about the formation of a chromium diffusion layer. Awide range of halides for incorporation in the layer is suggested, ironhalides being stated to be the most preferred.

Unfortunately we have found that when many of the halides suggested inthat application are employed in processes for chromizing ferroussubstrates, having a low free carbon content either by virtue ofdecarburization or by being stabilized by the addition of acarbide-forming element there is a tendency for the chromium-containingmetal powder applied to the substrate to sinter together to form anunsightly surface on the final diffusion product. It is necessary toremove this sintered coating by, for example, the use of wire brushes.

We have now found that this problem can be mitigated if eiter a hydratedaluminum halide or a hydrated magnesium halide or magnesium oxyhalide isemployed in the process.

Accordingly, the present invention provides a process for the chromizingof ferrous metal substrates having a low free carbon content ashereinafter defined which comprises:

(1) applying an adhesive layer to at least one surface of the substrate,optionally comprising or consisting.

When employed herein the term having a low free carbon content should betaken to denote steels which A 3,778,297 Patented Dec. 11, 1973 haveeither been decarburized, for example, by treatment with wet hydrogen orcontain a carbide-forming element such as titanium or niobium to reducethe carbon available in the steel for migrating during the diffusionprocess, such steels will contain no more than 0.01% of unbound carbon.

The process of the invention is particularly suitable for use inprocesses in which the diffusion stage is carried out with coatedsurfaces of the substrate in contact either with another coated surfaceor an uncoated surface of steel. Thus the process enables the diffusionstage to be carried out with the substrate arranged in the form of aclose coil or a stack of steel plates. The process can of course beemployed alternatively in the case of an open coil or where there is nocontact with adjacent substrates. We have found in the case whereadjacent surfaces are placed in contact that it is unnecessary to applythe adhesive layer and the chromium-containing metal powder to both ofthe surfaces in order to obtain a satisfactory diffusion coating on bothsurfaces, but that application to one side only is sufficient.

Substrates suitable for use in the process of the invention areparticularly substrates in which the carbon contained in the substratehas been bound by a carbideformer. The process of the invention isparticularly suitable for titanium-stabilized steels. The form ofsubstrate for which the process is most particularly suitable is steelstrip or plate.

The hydrated aluminum, magnesium halides or magnesium oxyhalidesemployed for the process of the invention are the chlorides oroxychlorides. We have found that these halides can, if applied from asolution in a volatilizable solvent, preferably water, be employed asthe adhesive layer onto which the chromium-containing metal powder isdeposited. However the halides may also be applied as dr solids. If drysolids are employed they may be applied before with, or after thechromium-containing metal powder. Magnesium oxychloride can be appliedeither as a solid or as an aqueous slurry. If desired, however, someother adhesive may also be employed and if this is the case the halideneed not be applied from a solution. For example, halide powder can beapplied to the substrate after the adhesive layer and thechromiumcontaining metal powder. However, it is preferred to apply thehalide first from an aqueous solution. If this is done the halide canconveniently be applied by passing the substrate, particularly when itis in the form of a strip, through a bath of an aqueous solution of thehalide.

The concentration of the halide in the volatilizable liquid solvent, ifthis is employed, may vary over wide ranges, such as, for example, fromat least 10-100 g./l. up to 5001,000 g./l., or up to the amount requiredto produce a saturated solution of the halide. In general, it is onlynecessary that the halide be present in a concentration to assure thatthe quantity of liquid to be applied on the substrate surfacewilldeposit the desired amount of the halide upon evaporation. Thus, it willbe largely determined by the volume of volatilizable liquid which can beevaporated in the drying stage without forming blisters or otherimperfections in the coating. However the halide or oxyhalide isapplied, it will be employed in an amount so as to provide approximately1-10 or 1-20 grams, and preferably about 2-5 grams of the halide oroxyhalide per square foot per side of surface area to be wetted. Thehalide or oxyhalide may be applied to only one or to both surfaces ofthe sheet material. If a solution is used this may be applied to oneside only by spraying or other suitable techniques such as by usingwetted rolls. In instances where the solution is applied to bothsurfaces, then the sheet material may be immersed in a body of thesolution, followed by withdrawing the wet sheet material from thesolution and passing it between squeege or metering rolls to removeexcess liquid. The substrate surface should be uniformly wetted with athin film of the solution without pooling or run off of liquid for bestresults, and preferably the solution is applied on one or both sides ofthe substrate by spraying a controlled amount of solution or bycontacting with wetted grooved rolls having a controlled amount of thesolution thereon. In instances where the solution is too dilute to applythe desired amount of the halide in one application, then a plurality ofapplications may be made followed by evaporation of solvent betweenstages, with the exception of the last stage as the sheet materialshould be wetted with the solution during the application of themetallic chromium-containing material in the coating step which follows.Applying the halide to both sides of the substrate permits bettercontrol and overcomes many practical problems of application. Moreoverit may result in an improved product especially in cases where thechromiumcontaining material is applied to one surface only. More dilutesolutions may be used to apply a given weight of halide per unit weightof metallic chromium without the problem of run off or pooling of thesolution discussed above, or blistering of the coating during drying.Higher weight ratios of halide to metallic chromium may be obtained, andthe more dilute halide solutions that are required to achieve a givenweight ratio are easier to work with.

It is of course highly desirable that the solution used to apply thehalide is not so acid as to result in attack on the sheet substrate. Wehave found that this may occur if either magnesium or aluminum halidesare employed and is particularly a problem if the halide is aluminumchloride. In such cases satisfactory solutions can be obtained byincorporating sufficient basic material which does not itself affect thechromizing reaction to raise the pH to more than 1.2. Suitable compoundsinclude alkali metal hydroxides and carbonates, such as sodium andptassium hydroxides and carbonates and alkaline earth metal oxides,hydroxides and carbonates such as magnesium and calcium oxides,hydroxides and carbonates. We have found the addition of magnesium oxideor carbonate is a particularly suitable method for this purpose and thatsolutions of aluminum chloride containing either of these having a pH inthe range 1.22.5 perferably from 1.7 to 2.3 are particularly suitable.If alkaline earth metal oxides are employed then these will be added assolids to the coating. Normally such addition will be after a solutionof the halide has been applied. In the case where magnesium chloride isemployed with magnesium oxide these may react together to form magnesiumoxychloride in situ.

Th'e source of metallic chromium may be commercially pure chromium, orchromium alloyed with metals which do not have an adverse affect on thechromizing process. Ferrochromium is usually preferred and for bestresults it should have a carbon content of 0.05% or less. The metallicchromium content of the source material should be at least 20%, and forbetter results at least 50%. In instances where ferrochromium isemployed, the chromium content is preferably at least 70% andferrochromium containing about 72-84% chromium is very satisfactory.

The source of metallic chromium may be commercialand the particles areof a size useful in the selected method of application. While a numberof methods of application are suitable, it is usually preferred tocontact thesubstrate surface with a gaseous suspension of the I '43; ,3,Pat. No. 3,090,353 and the patents mentionedhereinbefore. In instanceswhere the substrate surface is contacted with an aerosol of the chromiumcontaining particles, then the patricle size (Tyler screen) should notbe greater than about number five mesh, and preferably should not begreater than about number 30 mesh. Commercially available particleshaving a Tyler screen size between approximately minus 30 mesh and minus350 mesh, preferably about minus -200 mesh, and for best results minus.150- 200 mesh are very satisfactory and. are practical particle sizeswhich are useful for producing uniform coating by well known prior artelectrostatic deposition processes.

The chromium-containing material is deposited or the ferrous metalsubstrate in an amountto provide a desired weight of metallic chromiumper unit of surface area. In instances where the source has a relativelylow chromium content, the amount of the deposited coatingis adjustedaccordingly to provide the desired weight of metallic chromium. As ageneral rule, the metallic chro mium content of the coating should be atleast 5 grams per square foot of surface area to be chr'omized andpreferably at least 9 to 10 grams. Better results usually are producedwhen the coating contains about 11-15 grams of chromium per square footof surface area to be chromized and the coating weight may be increasedas desired up to the upper limit, which is approximately 35 to'50 gramsof metallic chromium per square foot of surface area to be chromized.The weight ratio of chromium to halide is normally from 1:1 to 10:1preferably from 2:1 to 5:1. It is understood that the coating weightsare given on a per side basis and are based on the metallic chromiumcontent thereof.

The particulate source of chromium is applied to the substrate while itis still wet or tacky with the solution of halide and/ or binder, andpreferably immediately after application of the solution. When appliedin this manner, the solution acts as a temporary binder for the metalparticles. The particles are deposited and retained on the wet surfacein the formof a uniform particulate chromium coating, and a more uniformchromized layer is produced on the substrate.

The liquid content in the particulate coating may be removed by heatingat an elevated temperature. This may conveniently be accomplished bypassing the coated substrate through an oven maintained at a temperaturehigh enough to cause rapid evaporation of the liquid and preferablyabove the boiling point for a sufiicient period'of time to dry thesubstrate surface. When water is the liquid, a temperature of about -l75C.and preferably about -155 C. is satisfactory and the substrate may beheated for up to about'l5 minutes normally from 1 to 60 seconds,preferably 5 to 30 seconds to ensure substantially complete evaporationof the water and loss of at least some water of hydration when presentin the halide. The removal of the liquid produces an adherentparticulate coating on the substrate containing the halide and thesource of chromium. The dried coated surface may be coiled or passedunder a roll without substantial loss of the particulate chromiumcoating and a compacting step is not necessary. I

Substrates having coated surfaces can be arranged for the heat treatmentin any convenient manner. For example, they may be arranged in stacks ordisposed separately in a furnace. Lengths of steel strip can be"'c oiledin either open or closed coil'manner.

Preferably, however, a plurality'of layers of dried coated sheetmaterial are assembled for chromizing the surfaces thereof with theadjacent layers or convolu'tions of the sheet material having at leastone'adherent particulate chromium coating therebetween. instances wherethe sheet material is in the form of discrete sheets, this may beconveniently accomplished by stacking the dried coated sheets insuperimposed relationship without compacting the coating. When the sheetmaterial is in the form of continuous strip, the dried coated strip iscoiled without compacting the metallic chromium-containing particles inthe coating. The strip may be coiled under a line tension ofapproximately 50-200 pounds per inch of width of the strip, and thisamount of coiling is satisfactory without resulting in compacting ordeforming of the particles in the coating to any substantial extent.

The assembly prepared from the dried coated sheet material is subjectedto a heat treatment cycle under conventional chromizing conditions in aprotective atmosphere. Preferably, the assemblies are placed in a closedvessel which is provided with an exhaust conduit and conduits forsupplying desired gases thereto to purge air from the vessel withnitrogen or an inert gas, and to maintain a protective atmosphere.Heating means is provided for maintaining the vessel at a desiredtemperature over the heat treatment cycle. In one suitable heattreatment cycle, air is replaced with nitrogen, then the nitrogenatmosphere is replaced with a protective atmosphere including hydrogenor a mixture of hydrogen and inert gas, and the assemblies are heated toapproximately 37 -425 C. and preferably to about 400 C. while passingthe protective atmosphere through the vessel to remove volatilematerials. This temperature may be held for approximately -20 hours andpreferably for about hours. After the purging is completed, thetemperature is raised to approximately 840-l,0l0 C. and preferably toabout 940-950 C., and this chromizing temperature is held for asufiicient period of time to chromize the sheet metal surfaces. Forexample, the chromizing temperature may be held for 10-80 hours andpreferably about -40 hours. During this period of time, the vessel isnot purged and the atmosphere is maintained at a positive pressure ofl-2 inches of water. The atmosphere in the vessel may be pure hydrogenan inert gas such as argon or helium or a mixture of an inert gas andhydrogen. During the chromizing step, the halide provides halogenbetween the adjacent layers of sheet material in the assembly andespecially next to the sheet material. As is well understood in the art,the halogen aids and promotes the chromizing of the adjacent surfaces ina minimum period of time.

After the chromizing step has been completed, the temperature is loweredto approximately 340-400 C. or below and the hydrogen-containingprotective atmosphere may be replaced with gaseous nitrogen. Afterreducing the temperature still further to approximately 150-210 C. orbelow, the furnace may be opened and the chromized sheet material isremoved. The chromized sheet material is subjected to washing with watersprays and/or is contacted with mechanically driven brushes to removeresidual chemicals and inert filler when present. Thereafter, thechromized sheet material may be brushed or given other mechanicaltreatment to produce a lustrous finish, or it may be temper rolled.

The process of the invention may be readily adapted to the operation ofcontinuous strip coating lines and especially high speed lines whichoperates at strip speeds of several hundred feet per minute and higherwherein the critical treatments to produce the adherent coating areperformed while the strip travels in a substantially straight line. Thestrip is continuously uncoiled and is passed through successive zonesfor wet cleaning the strip, drying the cleaned strip, applying asolution of an halide and/or binder on the top surface of the driedstrip and preferably also on the bottom surface, electrostaticallydepositing the particulate source of chromium on at least the topsurface of the wet strip and if desired also on the under surface,drying the coating to form an adherent particulate coating and thencoiling. The strip is preferably passed horizontally through theelectrostatic deposition zone and through the oven without beingcontacted by a roll on the chromium-coated side, whereby the particulatechromium coating on the upper surface is not disturbed prior to dryingthe solution and coiling.

In instances where the adherent powdered chromium coating is appliedonly to the upper surface of the strip, surprisingly it is stillpossible to chromize both surfaces. This is achieved without the needfor a filler for spacing material between adjacent convolutions of thestrip as there is no tendency for the adjacent sheets to adhere. Whendesired, open coil annealing may be employed for the chromizing stepwhen the substrate is coated with chromium on one or both sides.

The process of the invention is illustrated by the following examples:

COMPARATIVE EXPERIMENT Some pieces (6 ins. x 5 ins.) of titaniumstabilized 20 gauge steel of free carbon content less than 0.01% weredegreased in an alkaline degreasing bath, pickled in nitric acid, washedwith water and dried. One surface of each of the pieces of steel wasthen treated with a solution of ferrous chloride tetrahydrate (4 parts)in water (5 parts). The amount of solution applied was such that, afterdrying, 4 grams per sq. ft. of steel surface of solid remained. To thewet surface, ferrochromium powder (200 RS. Mesh) containing 83% chromiumwas evenly applied at the rate of 12 grams of chromium per sq. ft. ofsteel surface. The composite was dried at approximately C. and afterdrying it was found that the ferrochromium coating was adherent andcould not be readily removed by rubbing with the fingers. The pieces oftreated steel were then stacked together so that a coated side was incontact with the uncoated side of the adjacent piece of steel, and thestack was then bolted tightly between two heavy gauge steel end platesin order to simulate the conditions which would occur in a closed coilof steel. The pack was then placed in a suitable furnace from which theair was removed by purging with gaseous nitrogen. After removal of theair, the nitrogen atmosphere was replaced with a hydrogen atmosphere andthe furnace heated to 400 C. The flow of gas was continued through thefurnace for 10 hours to ensure the complete removal of harmful products.The flow of gas was then stopped and the temperature was raised to 950C. and

maintained at this temperature for 16 hours.

After cooling, the pieces of steel were separated from one another andwashed in water. The alloy diffusion layers contained an average of23.5% chromium and were 0.002 in. thick. However, the surfaces of thetreated steel were extremely rough and had sintered on them particles offerrochrome which could not be removed by brushing.

Example 1 An experiment was conducted identical to that described in thecomparative experiment, apart from the fact that the ferrous chloridesolution was replaced by a solution containing aluminum chloride, whichhad added to it magnesium carbonate sufiicient to give the solution a pHof 1.7. The amount of solution applied was suflicient to give rise to 4grams of aluminum chloride per sq. ft. of steel surface after drying.

A stack of pieces of steel was prepared and furnaced in the waydescribed in the comparative experiment.

Examination of the pieces of steel after furnacing revealed very littlesintering of the ferrochrome to the surfaces and the diffusion coatingsso obtained contained an average of 20.5% chromium and were 0.0023 in.thick.

Example 2 An experiment similar to that described in Example 1 wascarried out, except that the ferrous chloride solution was replaced by asolution of magnesium chloride of pH 5.2. The amount of solution appliedwas such that, after drying, 4 grams of a solid magnesium chloride persq. ft. of steel surface would remain.

A stack of plates was constructed and furnaced in the way described inExample 1. On examination of the pieces of steel after furnacing, it wasrevealed that little sintering had taken place, and the diffusion alloylayers which 7 had been formed contained an average chromium content of22.2% and had a thickness of 0.0021 in. We claim:

1. In a process for chromizing ferrous metal substrates which comprisesapplying an adhesive layer to the substrate, subsequently applyingchromium-containing powder thereto and then subjecting the substratehaving the chromium-containing powder thereon to a heat treatment in thepresence of a halide so as to bring about diffusion of the chromium intothe substrate, the improvement which comprises employing as the halideat least one halogen-containing substance selected from the groupconsisting of hydrated magnesium halide, aluminum halide and magnesiumoxyhalide, the said halogen-containing substance being applied to thesubstrate from an aqueous medium and a pH modifier being employed inconjunction with the said halogen-containing substance, the pH modifierbeing capable of reducing the acidity of the said halogen-containingsubstance and being employed in an amount to reduce the acidity thereof.

2. A process as claimed in claim 1 wherein the said halogen-containingsubstance is applied to the substrate from an aqueous medium prior tothe application of the chromium-containing powder so as to form the saidadhesive layer.

3. A process according to claim 1 wherein the said halogen-containingsubstance is applied to the substrate from an aqueous solution.

4. A process according to claim 3 wherein the pH modifier is present inan amount to adjust the pH of the solution to be within the range 1.2 to2.5.

5. A process according to claim 4 wherein the pH modifier is selectedfrom the group consisting of alkali metal hydroxides, alkali metalcarbonates, alkaline earth metal oxides, alkaline earth metal hydroxidesand alkaline earth metal carbonates.

6. A process according to claim 5 wherein aluminum chloride is employedin conjunction with magnesium carbonate.

7. A process according to claim 1 wherein the said halogen-containingsubstance is magnesium oxychloride.

8. A process according to claim 1 wherein the said halogen-containingsubstance is applied to the substrate in an amount of from 2 to 5 gramsper sq. ft.

9. A process according to claim 1 wherein said halogencontainingsubstance is applied from an aqueous solution prior to the applicationof the chromium-containing powder and provides the adhesive layer towhich the chro mium-containing powder is applied.

10. A process according to claim 1 wherein the substrate is arranged forthe heat treatment so that adjacent surfaces are in contact with eachother.

11. A process according to claim 1 wherein the chro- 8 mium-containingpowder is'applied to only 'one surface of the substrate. I i

12. A process according to claim 1 wherein the'chromium-containingpowder is applied at a rate equivalent to that required to'provide from-11 0515 grams of chromium per square foot to be chromized based on theen tire surface area to be chromized.

13. In a process forchromizing ferrous metal substrates which comprisesapplying an adhesive layer' to the substrate, subsequently applyingchromium-containing powder thereto and then subjecting the substratehaving the chromium-containing powder thereon to a heat' treatment inthe presence of a halide so as to bring about difiusion of the chromiuminto the substrate, the improvement which comprises employing as thehalide an'oxyhalide-of magnesium.

14. A process according to claim 13 wherein the oxyhalide is applied tothe substrate from an aqueous medium.

15. A process according to claim 13 wherein the oxyhalide is applied tothe substrate from an aqueous medium prior to the application of thechromium-containing powder so as to form the said adhesive layer.

16. A process according to claim 13 wherein the oxyhalide employed ismagnesium oxychloride.

17. A process according to claim 13 wherein the oxyhalide is applied tothe substrate in an amount of from 2 to 5 grams per sq. ft.

18. A process according to claim 13 wherein the substrate is arrangedfor the heat treatment so that adjacent surfaces are in contact witheach other.

19. A process according to claim 13 wherein the chromium-containingpowder is applied to only one surface of the substrate.

20. A process according to claim 13 wherein the chromium-containingpowder is applied ata rate equivalent to that required to provide from11 to '15 grams'of chromium per square foot to be chromized, based onthe entire surface area to be chromized.

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ALFRED L. LEAVITI, Primary Examiner

